This application relates to a transparent article having a metallic flake appearance. Specifically, this application relates to an article that comprises a pigment having a metallic flake appearance dispersed within a transparent thermoplastic matrix.
As thermoplastics become increasingly important in the marketplace, producers have sought to differentiate their products by making them more visually exciting to the customer. One desirable look for thermoplastics is the metal flake appearance. This appearance can be described as the look one could imagine if small pieces of metal were dispersed in molten glass, and the glass was subsequently cooled. The flakes have a metallic glint, but one perceives clear spaces between the metallic flakes such that portions of the article are completely transparent.
Until now, it has been very difficult to produce a thermoplastic article having a metal flake appearance. Metal flakes, most typically aluminum flake, have been dispersed in thermoplastics to create a metal flake appearance, but there are several disadvantages to this method. Specifically, the metal flakes tend to cause degradation of the polymer, which leads to a loss in mechanical properties. For instance, dispersing aluminum flake in polycarbonate will significantly decrease the impact strength of the polycarbonate. Moreover, aluminum flake is hazardous because it can cause dust explosions. If aluminum flake does burn in a fire, it burns at a very high temperature, which makes the fire very difficult to extinguish.
Mica has also been dispersed in thermoplastics in an attempt to create a metallic flake appearance. Mica can provide a metallic flake appearance, especially when coated with a layer of an inorganic pigment such as titanium dioxide. However, mica has several drawbacks. Specifically, incorporating mica in a transparent thermoplastic tends to decrease the transparency of the thermoplastic, producing a "milky" appearance. Mica also degrades the physical properties of the thermoplastic matrix. Finally, it is difficult to prepare materials containing mica platelets because excessive shear conditions may cause breakage of the platelets, which can ruin desired appearance.
The articles according to the present invention have a desirable metallic flake appearance, yet they avoid the above-identified problems with previous compositions having this same appearance.